1. Field of the Invention
This invention relates to the preparation of phenol-formaldehyde and melamine-formaldehyde resin-based binders modified with a urea-formaldehyde polymer containing at least 20% triazone and substituted triazone compounds (cyclic urea prepolymer) and to products prepared using the binders. More particularly, the invention relates to a prepolymer comprising urea, formaldehyde, and ammonia or a primary amine which, when added to a base resin, results in a useful binder or adhesive for the manufacture of numerous articles.
2. Description of Related Art
Phenol-formaldehyde resins and melamine-formaldehyde resins are standard resins used for many products. The choice of resin depends on the desired properties. Phenol-formaldehyde resins are strong and durable and relatively inexpensive, but are generally colored resins. Melamine resins are water clear but are more expensive. Hence they are generally used only for products whereby the color or pattern of the substrate is maintained with a transparent melamine protective coating or binder.
Phenol-formaldehyde Resins
Phenol-formaldehyde resins are used to make a variety of products including consolidated wood products such as plywood, engineered lumber, hard board, fiber board, oriented strand board, and other products such as fiberglass insulation, laminates, abrasive coatings, friction binders, foams, foundry binders, and petroleum recovery binders. They are also used as paper saturating resins for oil filters, overlay, paint roller tubes and the like.
Insulation
Insulation is generally prepared by coating glass fibers or mineral wool fibers with an aqueous binder solution, usually by spraying, and passing the coated fibers through an oven where they are compressed to the desired thickness and density, and then permanently fixed by heat setting or curing the resin binder. The traditional binders used in manufacturing insulation are low molecular weight, alkaline catalyzed phenol-formaldehyde resins fortified with formaldehyde scavengers, acid catalysts, and coupling agents. Acid cure has been favored in the art because it produces a glass fiber insulation having good strength and moisture resistance characteristics.
It is often desirable to scavenge the free formaldehyde prior to application. This is done for several reasons: 1) to reduce the free formaldehyde emissions during the forming and curing of the insulation product, 2) to reduce the free formaldehyde prior to the addition of an acid catalyst, 3) to reduce the cost of the binder, and 4) to improve the anti-punk properties of the resin.
The most common scavengers are chemical species containing a primary or secondary amine functionality. Urea, ammonia, melamine, and dicyandiamide are a few of the more commonly used amines. The most common, and the most economical, amine species is urea. When urea is used as the formaldehyde scavenger, the amount of urea added to the resin is referred to as the extension level which is reported as a percent of the binder solids. Binder solids consist of phenol-formaldehyde resin solids and extender solids.
The addition of formaldehyde scavengers to a phenol-formaldehyde resin requires a finite period of time to achieve equilibrium with the free formaldehyde in the resin. The process of reaching this equilibrium is referred to as pre-reaction, and the time to reach the equilibrium is referred to as the pre-react time. Pre-react times vary with temperature and amine species. When urea is used, the pre-react times range from 4 to 16 hours depending on temperature.
The mole ratio of formaldehyde to formaldehyde scavenger is also important and conditions are optimized to achieve the best performance of the binder. When urea is used, the mole ratio of formaldehyde to urea, the F/U, is optimally maintained between 0.8 and 1.2. If the extension level results in an F/U of less than 0.8, the opacity increases significantly along with the ammonia emissions. If the extension level results in an F/U of greater than 1.2, formaldehyde emissions increase and the risk of precipitation of dimethylolurea is greatly increased. For this reason, in traditional binders using urea as the formaldehyde scavenger (or a combination of urea and ammonia), the extension level is dictated by the amount of free formaldehyde in the base resin.
There are however disadvantages of pre-reacting resins with urea prior to forming the binder. Because free formaldehyde stabilizes the tetradimer in the resin, pre-reacting with urea will reduce the % free formaldehyde in the resin, hence reducing resin stability over time. Further, long pre-react times, as observed when urea is used as the formaldehyde scavenger, shorten the shelf life of the binder. In addition, pre-reacting with urea takes time, requires pre-react tanks and binder tanks, and urea needs to be stored in heated storage tanks. There is a need for an extender system for phenol-formaldehyde resins that does not have the above disadvantages.
Plywood and Engineered Lumber
It is also well known to use phenol-formaldehyde resins and phenol-formaldehyde resin-extenders and fillers as plywood and engineered lumber adhesives in the industry. Many raw materials may be added to phenol-formaldehyde resins to improve the bond qualities of the adhesive in plywood panels and engineered lumber such as laminated veneer lumber, including borax, potassium carbonate, poly-vinyl alcohol, etc. Urea has also been added to plywood and engineered lumber resins and adhesives to improve pre-press tack, bond quality, cost, assembly time tolerance, and reduce formaldehyde emissions without detrimental effects to the bonding strength of the adhesive. Urea can be added to the phenol-formaldehyde resins up to a 5% level based upon the solid weight of urea to the total resin weight at a 41% solids which includes the urea. However, when urea is used at high levels of 4 to 5 wt %, the phenol-formaldehyde resin selected must have a long assembly time (time between application of the adhesive and when the panels are hot pressed or pre-pressed), to eliminate dryout of the adhesive. Therefore, the use of urea in plywood resins is generally limited to levels lower than 5%, generally equal to or below 3%.
Oriented Strand Board
Spray dried oriented strand boards (OSB) and wafer board resins are very sensitive to any extender or filler that is used in the resin. Many attempts have been made to use small amounts of urea or urea-formaldehyde resins as extenders in various phenol-formaldehyde and phenol-melamine-formaldehyde resins. Unfortunately, most of these attempts to extend the resin are not commercially successful because the urea interferes with the ability of the resin to be spray dried. Urea contained in the phenol-formaldehyde resins for OSB or wafer board applications are typically limited to 1% urea for scavenging free formaldehyde. Otherwise the urea will affect the properties of the OSB wafer board such as its durability.
High Pressure Laminating Resins
Phenol-formaldehyde resins used to manufacture high pressure laminates are typically produced by reacting phenol and formaldehyde using an alkaline catalyst such as sodium hydroxide. Typical mole ratios of formaldehyde to phenol range from 1.0 to 2.5 moles of formaldehyde per mole of phenol with the preferred range from 1.2 to 1.9 moles of formaldehyde per mole of phenol. Catalyst levels range from 0.2% to about 6%, preferably from 0.5 to 3%. These materials are reacted to a suitable endpoint, cooled with vacuum, and usually distilled to remove the water present from the formaldehyde solution as well as the water of condensation from the polymerization reaction. They may be used in this state or an organic solvent such as methanol can be added to reduce the % solids and viscosity and aid in penetration of the kraft paper substrate. Modifiers such as urea can be added to reduce residual free formaldehyde levels. Other modifiers may also be added to achieve specific purposes.
High Pressure Laminates are made from several layers of paper that have been impregnated with thermosetting resins, dried (B-staged), and finally cured under pressure in a heated press. The surface of the laminate is made from a decorative paper (a solid color or printed with a pattern) that is impregnated with a melamine-formaldehyde resin. Underneath this surface are several layers of kraft paper that have been impregnated with a phenol-formaldehyde resin and function as a core for the laminate. Both the resin impregnated decorative paper and the resin impregnated kraft core paper are passed through ovens to increase the molecular weight of the resin component, and reduce the volatile level in the sheet (B-staging). After B-staging, a decorative sheet is laid up with several layers of the kraft core paper and loaded into a press. The press is brought up to pressure, typically 1000 psi, and then heated up to temperatures typically ranging from about 120.degree. C. to 160.degree. C. for 20 to 60 minutes. This is done to consolidate the multiple paper layers and cure the resin components. At the end of that time period the press is cooled and finally the pressure is released.
Some laminates are produced whose primary use is for flat surfaces. Other laminates are produced that are post-formed (thermoformed) into more complex shapes after the above pressing process is complete. These laminates are used for counter tops where the front edge is formed into a lip and the back edge is formed up into a back-splash area. The postforming laminates are usually under cured in the original pressing cycle or use a very formable (soft) resin. If the laminates are fully cured or utilize a stiffer more brittle resin, when they are postformed the laminates will crack and break This makes an unacceptable product for consumers. Brittle laminates also tend to chip and break when they are cut to size or machined prior to use or can be more breakage prone during installation and use. This is also unacceptable to the consumer.
Another problem in the laminating industry is the release of volatile organic components into the atmosphere during the B-staging process. One of these volatile organic components is phenol. Typical levels of free phenol in the phenol-formaldehyde resin used to impregnate the kraft core paper are in the 5-15% range. One method to reduce the free phenol level in the base phenol-formaldehyde resin is to increase the amount of formaldehyde (relative to the phenol) in the resin as manufactured. Unfortunately this usually results in a more brittle resin that when cured is unacceptable for manufacturing postforming laminates.
Paper Saturating Resins
Saturating resins, without modifiers, for oil filter, overlay, and for paint roller tube applications are typically low mole ratio resins in the range of 0.8-1.7 F/P. The low mole ratio resins give the treated paper more flexibility for pleating before curing. They are base catalyzed resins and are usually high molecular weight resins which are water insoluble. A distillation step is required and then the distilled resin is solvated in an alcohol--such as methanol, isopropanol, or ethyl alcohol. The resin is usually neutralized to a pH of 6.5-7.5 with acid to give lighter color cure. The resin is then applied to base paper, usually in dip roller pans, and then the treated paper goes into an oven to drive off the solvent, resulting in "B" staged paper. This paper is then rolled and shipped to the oil filter manufacturers. They then pleat and cut the paper and then it is cured in an oven. The cured paper will then have oil, temperature, water, and chemical resistant properties. Saturating resins for plywood overlays work in a similar way, except the treated paper is not pleated but is bonded onto plywood or other substrate with heat and pressure, which cures the resin.
There are some high mole ratio saturating resins, typically in the range 1.8-2.5 F/P which are water soluble. These, however, must be modified with a plasticizer such as a thermoplastic latex to give the treated paper pleatability. The high mole ratio resins alone will be too high in cross link density and therefore brittle when cured. The advantage in waterborne resins are no emissions from solvent and due to higher F/P mole ratios there will be less emissions of free phenol.
Other Uses
Phenol-formaldehyde foam resins are used to make open or closed cell foams when cured. Such foams are primarily used to make floral foam supports for supporting flower stems in water. The foam is able to soak up water many times it's weight to provide water for the flowers. Such foams are primarily open cell foams with perforations in cell walls. Other uses for phenol-formaldehyde foams are dense foams used for models similar to balsa wood, foam to hold jewelry, foam used to make molds for foot prosthetics and closed cell foam for barrier and insulation type properties.
Other uses of phenol-formaldehyde resins include abrasive binders, friction binders, and phenol-formaldehyde coated foundry sand binders.
Melamine-Formaldehyde Resins
Melamine-formaldehyde resins provide binders that are clear. Hence such resins are suitable for products such as ceiling tiles, paper laminates (e.g., veneer for countertops), and molded articles. However, currently there is a shortage of melamine crystal used in manufacturing the melamine-formaldehyde resins. In addition, melamine crystals are expensive.
Acoustic ceiling tiles are presently back-coated with melamine resins in order to make them more rigid and humidity-resistant when installed in suspended ceilings. Melamine resins are also used for the preparation of decorative or overlay paper laminates due to their excellent color, hardness, and solvent, water, chemical resistance, heat resistance and humidity-resistance.
Molded articles, such as dinnerware, are presently prepared with a combination of melamine-formaldehyde resins and urea-formaldehyde resins. The resins are combined because the melamine-formaldehyde resin is too expensive to use by itself. However, such articles are generally not very water-resistant or dimensionally stable.
It would be beneficial to provide an extender for both phenol-formaldehyde resins and melamine-formaldehyde resins in order to reduce formaldehyde emissions, phenolic emissions, improve properties of the products obtained with the resins, and to reduce overall cost of the resins.